Since a report of Wilkes et al. in 1992, an ionic liquid attracts attention as a substance which is a liquid at normal temperature and excellent in an ion conductivity. In the ionic liquid, a cation is bonded with an anion via an electrostatic attraction, the number of the ion carrier is very large, a viscosity is relatively low and thereby an ion mobility is high even at normal temperature, so that the ionic liquid has a very high ion conductivity. Further, the ionic liquid has a high boiling point and a very wide temperature range in which it can remain in a liquid state, because it consists of the cation and anion. Furthermore, the ionic liquid is low in a flammability and very excellent in a thermal stability, because it has little vapor pressure (see J. Electrochem. Soc., 144 (1997) 3881 and “Function Creation and Application of Ionic Liquid”, N. T. S, (2004)).
The ionic liquid has these various merits, so that it is recently studied to apply to electrolytes for a non-aqueous electrolyte secondary battery and an electric double layer capacitor (see JP-A-2004-111294 and JP-A-2004-146346). In particular, when the ionic liquid is used in the electrolyte for the electric double layer capacitor, it also serves as an ion source for forming an electric double layer, so that there is also a merit that it is not necessary to add an additional supporting electrolyte.